Someone deletes my client director's credit from his feature page

Hello guys,

My client (I am an agent) keeps adding his name as one of four directors (as credited in the main titles and in the contracts) of a feature documentary and one of the other directors keeps deleting my client's name from the credit.

Here is the documentary page: https://www.imdb.com/title/tt32063216/?ref_=nv_sr_srsg_0_tt_8_nm_0_in_0_q_il%2520segreto%2520di%2520

How can we avoid the other guy to delete my client's name?

Thank you in advance for your help and all the best.